There are various methods of separating a component contained in fluids such as liquid and gas. For example, taking as an example techniques for removing ionic materials contained in sea water, brackish water, and the like, separation methods using an separation membrane element has been increasingly used in recent years as a process for energy saving and resource saving. Examples of separation membranes used in separation methods using a separation membrane element include, in terms of its pore size and separation function, a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, a reverse osmosis membrane, a forward osmosis membrane, and the like. These membranes are used in obtaining drinking water, for example, from sea water, brackish water, and water containing harmful substances and in the production of industrial ultrapure water, wastewater treatment, recovery of valuables, and the like and used appropriately depending on the desired component to be separated and separation performance.
In recent years, separation membrane elements suitable for purification of sewage (domestic wastewater deriving from cooking, laundering, a bath room, a toilet room, and other living environments) and wastewater deriving from a manufacturing plant, a restaurant, a fish processing plant, a food processing plant, and the like have been widely used in wastewater treatment because the treatment can be carried out in a small space.
The separation membrane elements used in these treatments have in common in that source fluid is fed to one surface of a separation membrane and permeate fluid is obtained from the other surface even if the source fluid to be treated varies. The separation membrane element is constituted such that a number of separation membranes with various shapes is bundled to increase membrane area per unit element to thereby obtain much permeate fluid per unit element. In addition, various elements have been produced according to the intended use and purpose, such as spiral-type, hollow-fiber-type, plate-and-frame-type, rotating flat membrane-type, and integrated flat membrane-type elements.
For example, as a fluid separation membrane element used in reverse osmosis filtration, a spiral-type separation membrane element comprising a water collecting pipe having a hollow portion and a unit comprising a feed spacer that feeds source fluid to a separation membrane surface, a separation membrane that separates components contained in the source fluid, and a permeate spacer for conducting permeate fluid that has permeated through the separation membrane and been separated from the feed fluid (the source fluid at the time when fed to the separation membrane element) to the water collecting pipe, wherein the unit is wound around the water collecting pipe, is widely used because permeate fluid can be taken out in large amounts by putting pressure on the source fluid.
As the feed spacer, a polymer net is mainly used in order to form a flow path for feed fluid (hereinafter referred to as a feed flow path). As the separation membrane, a composite semipermeable membrane in which a separation functional layer mainly composed of crosslinked polymers such as polyamide, a porous support layer composed of polymers such as polysulfone, and a nonwoven fabric composed of polymers such as polyethylene terephthalate, which nonwoven fabric serves as a substrate, are each laminated from the feed side to the permeate side is used. Further, as the permeate spacer, a fabric called a tricot having a narrower interval than that of the feed spacer is used in order to prevent sagging of the membrane and form a flow path for permeate fluid (hereinafter referred to as a permeate flow path).
In recent years, reduction in the cost of water production has been increasingly expected, and accordingly separation membrane elements with higher performance have been demanded. To increase separation performance of a separation membrane element and the amount of permeate fluid per unit time, improvement in performance of each flow path member, separation membranes, and element members has been proposed. For example, in Patent Document 1, the method that uses a sheet the surface of which is shaped into an uneven shape as a permeate spacer has been proposed. In Patent Document 2, the method that does not use a substrate such as a permeate spacer or a feed spacer and uses a flat membrane on the feed surface of which great unevenness is formed and inside which a hollow passage is provided has been proposed. In Patent Document 3, the method that uses a sheet-like composite semipermeable membrane in which a porous support layer having great unevenness on its surface is provided on a nonwoven fabric and a separation active layer is further provided thereon and that does not use a feed spacer such as a net or a permeate spacer such as a tricot has been proposed.
In addition, in the treatment of wastewater such as sewage, for the reduction in the cost of wastewater treatment by a separation membrane element, there is a growing need for water permeability and durability of the membrane element. Thus, as shown in Patent Document 4, a separation membrane element having high water permeability, wherein the rejection rate hardly decreases even when the surface of the separation functional layer is worn; a porous support layer (separation functional layer) is not readily peeled off from a porous substrate such as a nonwoven fabric; and further, even if clogging has once occurred, the clogging substance can be readily removed, has been proposed.